Transport heterogeneity in mammalian proximal tubules proposes to use the technique of in vitro microperfusion of isolated rabbit tubules to examine specific epithelial transport characteristics. These studies will basically examine whether significant quantitative and qualitative differences in transport of solutes and water exist both between the same tubule segments from different nephrons (internephron heterogeneity) or along the length of a given nephron (intranephron heterogeneity). Specific issues to be examined in early and late proximal tubules from superficial and juxtamedullary nephrons include: permeability coefficients of Na, Cl and HCO3; reflection coefficients for glucose, amino acids, NaCl and NaHCO3; amounts of NaCl, NaHCO3 and water reabsorbed; and the mechanisms behind NaCl and NaHCO3 reabsorption (active vs passive transport). The results obtained will be used to propose a model for overall proximal tubule transport.